1. Field of the Invention
This invention relates to helical spring assemblies and, in particular, to a spring holder utilized in such assemblies, which has a helically-threaded portion onto which a helically-wound spring can be mounted. The thread of the spring holder has a tension bearing side against which a mounted spring bears when it is in tension, and a compression bearing side against which the spring bears when it is in compression. Typically, the ends of a spring are mounted on a pair of axially-spaced spring holders which are utilized as a means for securely attaching the spring to apparatus such as reciprocating devices, where the spring is used to absorb vibrations.
The tensile and compressive forces produced by a spring mounted on a holder are a function of the number of spring coils which are active (free to move) after mounting. Expressions defining the magnitudes of forces produced by helical springs are well known. For example, see A. M. WAHL, MECHANICAL SPRINGS, p. 56 (Second Edition 1963) where the expressions listed below as equations 1 and 2 are presented. EQU P=k.delta. (Eqn. 1) EQU k=Gd.sup.4 /8D.sup.3 n (Eqn. 2)
In equation 1, P represents the tensile or compressive force produced by a spring when it is elongated or compressed, k is the spring constant and .delta. is the deflection from its relaxed length which the spring is elongated or compressed. The spring constant k is further defined by equation 2 where G is the shear modulus, d is the cross-sectional diameter of the spring wire, D is the diameter of the spring coils and n is the number of active coils. By substituting equation 2 into equation 1, the following equation is derived: EQU P=(Gd.sup.4 /8D.sup.3 n).delta. (Eqn. 3)
2. Description of the Prior Art
The thread on a conventional spring holder follows the pattern of a regular helix and terminates in a planar-shaped end of the spring holder. When a spring is mounted on the spring holder, the number of active coils in tension n.sub.t differs from the number of active coils in compression n.sub.c, causing different tensile and compressive spring constants. Thus there is a difference in the tensile force P.sub.t and the compressive force P.sub.c produced by the spring for tensile and compressive deflections of equal magnitude. The number of active coils in tension is greater than the number of active coils in compression, because the first coil engaging the thread of the spring holder contacts the compression bearing side of the thread before it contacts the tension bearing side, and thus a portion of this coil which is not restricted against elongation is restricted against compression. This difference in the tensile and compressive spring constants is significant unless a relatively large number of coils are active, and in some applications it can adversely affect operation of the spring. For example, if the spring is mounted in a reciprocating device, for harmonically absorbing vibration of parts moving at the reciprocating frequency of the device, the spring might be ineffectual and the unequal forces produced thereby could even cause an increase in vibration by unbalancing the moving parts.
In other applications it is desirable to arbitrarily establish predetermined ratios of the tensile and compressive forces produced by a mounted spring for specific deflections .delta. of the spring. With conventional spring holders this can not be done, however, because the ratio of the number of coils in tension and in compression cannot be arbitrarily established.
Conventional spring holders also tend to cause the tensile force produced by the spring to change non-linearly with respect to deflection during maximal elongations of the spring. It is known that this non-linear force is a result of the combined tensile force produced by the spring, which is itself designed for linear operation, and the counteracting force applied to the spring by the thread of the spring holder. The non-linearity is caused by flexing of the thread near its end, where the spring engages the spring holder. The end of the thread will yield and eventually fatigue if it is continually subjected to such flexing.